Kelvin measurement, also referred to as 4-wire measurement or remote voltage sense measurement, consists of measuring the resistance of a device under test (DUT) by inducing a known current flow into the DUT and reading the voltage drop remotely at the DUT. The accurate resistance measurement is then calculated from the separate voltage and current measurements using Ohms law, which is the ratio of voltage divided by resistance.
Refer to FIG. 1 for the following discussion of the manner in which Kelvin measurements are conducted. The current is provided to the DUT by two electrical leads, whose lead resistances are represented by RL1 and RL2. The ammeter can be modeled as a standalone current source. The current flowing through the RL1, RL2, and the DUT are all the same, as a result of these three devices being connected in series. The path of the current is represented by the line labeled “current loop 1.”
The parallel resistance of the voltmeter connected to the DUT is many orders of magnitude higher than the DUT, and so near zero current will flow into the voltmeter. In typical real world applications, the relationship of voltmeter impedance to DUT resistance is about 10 GΩ to 100 mΩ, or a 1011 factor greater.
The two voltmeter electrical leads' resistances, which are represented by RL3, and RL4, can be neglected because of the near zero current flow into the voltage meter.
As mentioned above, an accurate resistance measurement is calculated from the separate voltage and current meter readings. In a Kelvin circuit, the current measurement accuracy is dominated by the accuracy of the ammeter, and the voltage measurement accuracy is dominated by the accuracy of the voltmeter.
In general, most DUT's measured with the Kelvin method are purely resistive; as a result the current induced into the circuit is a constant current from a DC source. However, there are instances where the device impedance is the measurement that is being sought. Because device impedance is a function of frequency, the current is generated from an AC source, at the required frequency.
Micro Kelvin measurements are instances where the DUT and/or DUT contact surfaces are too small to attach conventional meter probes to it. The current methods for making micro Kelvin measurements all have limitations.
One method of making micro Kelvin measurements can be termed a “double trace” method. There are two separate fine traces on the test printed circuit board, one for each of the two individual surface contacts (typically gold pads) on the board to which the DUT is mated via a temporary contact, called an “interposer.” In FIG. 2a, ACE material 17 is illustrated as the interposer between the two interconnection surfaces (i.e. gold pads) 12, 14 on the surface of the board, and contacts 16, 18 of the DUT. FIG. 2b illustrates the electrical equivalent circuit formed by FIG. 2a. Ammeter 13 and voltmeter 15 are shown. The resistance of the ACE interposer has been represented by RINT1, and RINT2. The circuit of FIG. 2b has been redrawn in FIG. 3 to illustrate a flaw of this design. As seen in FIG. 3, the flaw that the resistances of the connector, represented as RINT1, and RINT2, add to the measurement of the DUT, because the voltage drop measurement is made at the outer nodes of the connector resistances. This introduces error.
A second method is termed a “double contact” method. See FIGS. 4a and 4b. There are four separate small contacts 20-23 on the surface of the test PCB (not shown), with a fine trace (not shown) leading to each contact. The two DUT interconnection surfaces or pads 24 and 26 are connected to pads 20, 21 and 22, 23, respectively, through a set of four separate mechanical contacts, one for each of contacts 20-23. These mechanical contacts can be spring-loaded pins or formed beam contacts; spring-loaded pins are shown in FIG. 4a. FIG. 4b illustrates the electrical equivalent circuit formed by FIG. 4a. The resistance of the contacts has been represented by RINTA, RINTB, RINTC, and RINTD. FIG. 5 is a top down view illustration of the relationship between all of the circuit elements of FIGS. 4a and b. 
To illustrate the improvement of this method over the double trace method mentioned above, FIG. 4d has been redrawn in FIG. 6. The advantage is that the current from the ammeter still flows through the resistors RINTA, DUT, and RINTC but now the voltage drop is measured across the nodes of the DUT. Also illustrated in FIG. 6 is that the contact resistances of RINTB, and RINTD are now part of the lead resistance of the voltmeter. As mentioned above, because almost no current flows through the voltmeter, the resistances RINTB, and RINTD are essentially irrelevant to the measurement.
The disadvantages of using the pins or formed contacts as interposers are:                Cost: Pin and formed contact technologies tend to be very expensive per interconnect, particularly with the fine pitches so prevalent today.        Inductance: Pin and formed contact technologies have high inductances at high frequencies, which is a problem when making Kelvin inductance measurements.        Inability to work at smaller pitches: Pin and formed contact technologies are physically relatively large, with physical limitations (such as metal forming technologies and spring fabrication) that dictate their minimum size and thus dictate the contact pitch with which they can be used.Anisotropic Conductive Elastomer        
Anisotropic Conductive Elastomer (ACE) is a composite of conductive metal elements in an elastomeric matrix that is normally constructed such that it conducts along one axis only. In general this type of material is made to conduct through its thickness. Anisotropic conductivity is achieved in one form of ACE by mixing magnetic particles with a liquid resin, forming the mix into a continuous sheet, and curing the sheet in the presence of a magnetic field. This results in the particles forming multiple separate but closely-spaced electrically conductive columns through the sheet thickness, each column separated from the others by cured insulating resin. Another group of ACE materials is constructed by embedding fine wire in a polymer matrix.
The resulting structure has the unique property of being both flexible and anisotropically conductive. These ACE materials can be constructed in large, continuous sheets, which can provide separable electrical interconnection over an extended area.